U.S. patent number 7,968,012 [Application Number 12/044,676] was granted by the patent office on 2011-06-28 for method and apparatus for emi shielding.
This patent grant is currently assigned to Laird Technologies, Inc.. Invention is credited to Jeff McFadden, Frank T. McNally, Martin L. Rapp.
United States Patent |
7,968,012 |
Rapp , et al. |
June 28, 2011 |
Method and apparatus for EMI shielding
Abstract
Disclosed are methods for manufacturing electromagnetic
interference shields for use in nonconductive housings of
electronic equipment. In one embodiment, the shield may include an
electrically nonconductive substrate, such as a thermoformable
film, coated with an electrically conductive element, such as an
extensible ink or a combination of conductive fibers with an
extensible film. In one embodiment, a compressible conductive
perimeter gap gasket may be formed by using a form in place
process.
Inventors: |
Rapp; Martin L. (Chesterfield,
MO), McFadden; Jeff (Rochester, NY), McNally; Frank
T. (Kennett Square, PA) |
Assignee: |
Laird Technologies, Inc.
(Chesterfield, MO)
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Family
ID: |
40026320 |
Appl.
No.: |
12/044,676 |
Filed: |
March 7, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080283186 A1 |
Nov 20, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11287566 |
Nov 23, 2005 |
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Current U.S.
Class: |
252/511;
428/301.1; 428/316.6; 428/297.4; 252/500; 427/58; 428/308.4;
174/350; 174/358; 428/300.1 |
Current CPC
Class: |
H01B
1/22 (20130101); H05K 9/009 (20130101); B29C
70/508 (20130101); B29C 70/882 (20130101); B29C
70/02 (20130101); B32B 37/206 (20130101); B32B
2305/18 (20130101); Y10T 428/249951 (20150401); B32B
2553/00 (20130101); Y10T 428/249981 (20150401); B32B
2311/00 (20130101); Y10T 428/249958 (20150401); Y10T
428/24994 (20150401); Y10T 428/249948 (20150401); B32B
2309/105 (20130101) |
Current International
Class: |
H05K
9/00 (20060101); B32B 5/24 (20060101); B32B
3/26 (20060101); B32B 27/12 (20060101) |
Field of
Search: |
;252/500,511,512,514
;428/297,299.1,299.7,301,317,318,315,372 ;361/818
;106/1.25,1.18,31.92 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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EP 643551 |
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Mar 1995 |
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EP |
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59225927 |
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Dec 1984 |
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JP |
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01257047 |
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Oct 1989 |
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JP |
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Other References
GE News Release, PR #43-99, Oct. 6, 1999. pp. 1-2. cited by
other.
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Primary Examiner: Silverman; Stanley
Assistant Examiner: Vijayakumar; Kallambella
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. Reissue application Ser.
No. 11/287,566 filed Nov. 23, 2005, which claims priority to U.S.
patent application Ser. No. 09/795,669 filed Feb. 28, 2001, which
is a continuation-in-part of U.S. patent application Ser. No.
09/768,428 filed Jan. 24, 2001, which claims priority to U.S.
Provisional Application No. 60/185,597 filed Feb. 28, 2000.
Claims
What is claimed is:
1. A method comprising integrating conductive fibers into an
extensible film to form an extensible electrically-conductive
coating having a surface resistivity that increases no more than
twofold when the coating is drawn at least six-tenths of an inch,
wherein integrating the conductive fibers comprises carding, which
includes combing the conductive fibers into a non-woven mat of
non-conductive fibers with a toothed card.
2. The method of claim 1, wherein integrating the conductive fibers
comprises blending the conductive fibers with nonconductive
fibers.
3. The method of claim 2, wherein the method includes depositing
the blended mixture to a thermoformable film at a sufficiently high
temperature to at least partially melt the nonconductive
fibers.
4. The method of claim 2, wherein the method includes: laminating
the blended mixture to a thermoformable film; or calendering the
blended mixture to a thermoformable film; or processing the
extensible film into the nonconductive fibers.
5. The method of claim 1, wherein the electrically-conductive
coating has: a surface resistivity of about 0.5 ohms/square or less
in an unformed state, and a surface resistivity of about 1
ohms/square or less in a formed state following a draw of
six-tenths of an inch; and/or a shielding effectiveness of about 40
decibels at 10 gigahertz in an unformed state, and a shielding
effectiveness of about 42 decibels at 10 gigahertz in a formed
state following a draw of six-tenths of an inch.
6. The method of claim 1, wherein the electrically-conductive
coating has: a surface resistivity of about 0.09 ohms/square or
less in an unformed state, and a surface resistivity of about 0.13
ohms/square or less in a formed state following a draw of
six-tenths of an inch; and/or a shielding effectiveness of about 60
decibels at 10 gigahertz in an unformed state, and a shielding
effectiveness of about 60 decibels at 10 gigahertz in a formed
state following a draw of six-tenths of an inch.
7. The method of claim 1, wherein integrating the conductive fibers
comprises positioning conductive fibers between an outer surface of
an extensible film and an outer surface of a thermoformable film,
and laminating the extensible and thermoformable films with the
conductive fibers sandwiched therebetween such that the conductive
fibers are integrated with the extensible film.
8. The method of claim 1, further comprising applying the coating
substantially uniformly to at least one of a first side or a second
side of a thermoformable film.
9. The method of claim 1, further comprising selectively applying
the coating to at least one zone and not to another zone on at
least one of a first side or a second side of a thermoformable
film.
10. The method of claim 1, further comprising applying the coating
to at least a portion of at least one of a first side or a second
side of a thermoformable film.
11. The method of claim 10, further comprising thermoforming the
thermoformable film having the coating applied thereto.
12. The method of claim 11, wherein thermoforming includes positive
forming the thermoformable film having the coating applied thereto
into a three-dimensional shape.
13. The method of claim 10, further comprising mixing conductive
particles with foamable materials to form a foam mixture with an
integral network of conductive particles, and applying the foam
mixture to the thermoformable film.
14. The method of claim 10, further comprising selecting the
extensible film from a plurality of materials such that the
selected extensible film has a glass transition temperature lower
than the glass transition temperature of the thermoformable
film.
15. The method of claim 1, wherein integrating the conductive
fibers into the extensible film forms the extensible
electrically-conductive coating such that the coating's shielding
effectiveness changes no more than five percent after the coating
is three-dimensionally formed.
16. The method of claim 1, wherein integrating the conductive
fibers into the extensible film forms the extensible
electrically-conductive coating such that the coating's surface
resistivity increases no more than forty-four percent when the
coating is drawn at least six-tenths of an inch.
17. The method of claim 1, further comprising coupling a
compressible EMI gasket to the coating, and positioning the EMI
gasket and the coating relative to an electronic device housing to
shield against ingress and egress of electromagnetic energy
relative to the electronic device housing.
18. The method of claim 17, wherein the coating has a surface
resistivity of about 1 ohms/square or less.
19. The method of claim 17, wherein coupling the EMI gasket to the
coating includes applying the coating to a thermoformable film, and
applying the EMI gasket to the thermoformable film.
20. The method of claim 19 wherein applying the EMI gasket to the
thermoformable film comprises mixing conductive particles with
foamable materials to form a foam mixture with an integral network
of conductive particles, and processing the foam mixture with the
integral network of conductive particles to shape the EMI
gasket.
21. The method of claim 20 wherein processing the foam mixture
comprises moving the surface of the thermoformable film relative to
a nozzle supplying the foam with the integral network of conductive
particles to form the EMI gasket in place.
22. The method of claim 18 wherein the method includes integrating
the conductive fibers into the extensible film to form the
extensible electrically-conductive coating such that the coating's
shielding effectiveness changes no more than five percent after the
coating is three-dimensionally formed.
23. The method of claim 18 wherein the method includes integrating
the conductive fibers into the extensible film to form the
extensible electrically-conductive coating such that the coating's
surface resistivity increases no more than forty-four percent when
the coating is drawn at least six-tenths of an inch.
Description
FIELD OF THE INVENTION
This invention relates to methods of manufacturing electromagnetic
interference ("EMI") shields and the EMI shields produced
thereby.
BACKGROUND
As used herein, the term EMI should be considered to refer
generally to both EMI and radio frequency interference ("RFI")
emissions, and the term electromagnetic should be considered to
refer generally to electromagnetic and radio frequency.
During normal operation, electronic equipment generates undesirable
electromagnetic energy that can interfere with the operation of
proximately located electronic equipment due to EMI transmission by
radiation and conduction. The electromagnetic energy can be of a
wide range of wavelengths and frequencies. To minimize the problems
associated with EMI, sources of undesirable electromagnetic energy
may be shielded and electrically grounded. Shielding is designed to
prevent both ingress and egress of electromagnetic energy relative
to a housing or other enclosure in which the electronic equipment
is disposed. Since such enclosures often include gaps or seams
between adjacent access panels and around doors, effective
shielding is difficult to attain, because the gaps in the enclosure
permit transference of EMI therethrough. Further, in the case of
electrically conductive metal enclosures, these gaps can inhibit
the beneficial Faraday Cage Effect by forming discontinuities in
the conductivity of the enclosure which compromise the efficiency
of the ground conduction path through the enclosure. Moreover, by
presenting an electrical conductivity level at the gaps that is
significantly different from that of the enclosure generally, the
gaps can act as slot antennae, resulting in the enclosure itself
becoming a secondary source of EMI.
Specialized EMI gaskets have been developed for use in gaps and
around doors to provide a degree of EMI shielding while permitting
operation of enclosure doors and access panels. To shield EMI
effectively, the gasket should be capable of absorbing or
reflecting EMI as well as establishing a continuous electrically
conductive path across the gap in which the gasket is disposed.
Conventional metallic gaskets manufactured from copper doped with
beryllium are widely employed for EMI shielding due to their high
level of electrical conductivity. Due to inherent electrical
resistance in the gasket, however, a portion of the electromagnetic
field being shielded induces a current in the gasket, requiring
that the gasket form a part of an electrically conductive path for
passing the induced current flow to ground. Failure to ground the
gasket adequately could result in radiation of an electromagnetic
field from a side of the gasket opposite the primary EMI field.
In addition to the desirable qualities of high conductivity and
grounding capability, EMI gaskets in door applications should be
elastically compliant and resilient to compensate for variable gap
widths and door operation, yet tough to withstand repeated door
closure without failing due to metal fatigue, compression set, or
other failure mechanism. EMI gaskets should also be configured to
ensure intimate electrical contact with proximate structure while
presenting minimal force resistance per unit length to door
closure, as the total length of an EMI gasket to shield a large
door can readily exceed several meters. It is also desirable that
the gasket be resistant to galvanic corrosion which can occur when
dissimilar metals are in contact with each other for extended
periods of time. Very low resistance and, concomitantly, very high
electrical conductivity are becoming required characteristics of
EMI gaskets due to increasing shielding requirements. Low cost,
ease of manufacture, and ease of installation are also desirable
characteristics for achieving broad use and commercial success.
Conventional metallic EMI gaskets, often referred to as copper
beryllium finger strips, include a plurality of cantilevered or
bridged fingers forming linear slits therebetween. The fingers
provide spring and wiping actions when compressed. Other types of
EMI gaskets include closed-cell foam sponges having metallic wire
mesh knitted thereover or metallized fabric bonded thereto.
Metallic wire mesh may also be knitted over silicone tubing. Strips
of rolled metallic wire mesh, without foam or tubing inserts, are
also employed.
One problem with metallic finger strips is that to ensure a
sufficiently low door closure force, the copper finger strips are
made from thin stock, for example on the order of about 0.05 mm
(0.002 inches) to about 0.15 mm (0.006 inches) in thickness.
Accordingly, sizing of the finger strip uninstalled height and the
width of the gap in which it is installed should be controlled to
ensure adequate electrical contact when installed and loaded, yet
prevent plastic deformation and resultant failure of the strip due
to overcompression of the fingers. To enhance toughness, beryllium
is added to the copper to form an alloy; however, the beryllium
adds cost. Finger strips are also expensive to manufacture, in part
due to the costs associated with procuring and developing tooling
for outfitting presses and rolling machines to form the complex
contours required. Changes to the design of a finger strip to
address production or performance problems require the purchase of
new tooling and typically incur development costs associated with
establishing a reliable, high yield manufacturing process.
Notwithstanding the above limitations, metallic finger strips are
commercially accepted and widely used. Once manufacturing has been
established, large quantities of finger strips can be made at
relatively low cost.
Metallized fabric covered foam gaskets avoid many of the
installation and performance disadvantages of finger strips;
however, they can be relatively costly to produce due to expensive
raw materials. Nonetheless, EMI gaskets manufactured from
metallized fabrics having foam cores are increasing in popularity,
especially for use in equipment where performance is a primary
consideration.
As used herein, the term metallized fabrics include articles having
one or more metal coatings disposed on woven, nonwoven, or open
mesh carrier backings or substrates and equivalents thereof. See,
for example, U.S. Pat. No. 4,900,618 issued to O'Connor et al.,
U.S. Pat. No. 4,910,072 issued to Morgan et al.; U.S. Pat. No.
5,075,037 issued to Morgan et al., and U.S. Pat. No. 5,393,928
issued to Cribb et al., the disclosures of which are herein
incorporated by reference in their entirety. Metallized fabrics are
commercially available in a variety of metal and fabric carrier
backing combinations. For example, pure copper on a nylon carrier,
nickel-copper alloy on a nylon carrier, and pure nickel on a
polyester mesh carrier are available under the registered trademark
Flectron.TM. metallized materials from Advanced Performance
Materials located in St. Louis, Mo. An aluminum foil on a polyester
mesh carrier is available from Neptco, located in Pawtucket,
R.I.
The choice of metal is guided, in part, by installation conditions
of the EMI shield. For example, a particular metal might be chosen
due to the composition of abutting body metal in the enclosure to
avoid galvanic corrosion of the EMI shield, which could increase
electrical resistance and deteriorate electrical grounding
performance. Metallized tapes are desirable both for ease of
application as well as durability.
Metallized fabrics, such as those described in the O'Connor et al.
patent mentioned hereinabove, are generally made by electroless
plating processes, such as electroless deposition of copper or
other suitable metal on a catalyzed fiber or film substrate.
Thereafter one or more additional layers of metal, such as nickel,
may be electrolessly or electrolytically deposited on the copper.
These additional layers are applied to prevent the underlying
copper layer from corroding, which would increase the resistance
and thereby decrease the electrical conductivity and performance of
any EMI gasket made therefrom. The additional nickel layer on the
copper also provides a harder surface than the base copper.
SUMMARY OF THE INVENTION
Two developments have been progressing independently for several
years in the area of EMI shields for nonconductive enclosures, such
as molded plastic housings for cellular telephones, computers, and
the like. The first development is a form in place ("FIP") process.
See, for example, U.S. Pat. No. 5,822,729 entitled Process for
Producing a Casing Providing a Screen Against Electromagnetic
Radiation, the disclosure of which is incorporated herein by
reference in its entirety. A goal of the FIP process is to produce
a conductive and compressible elastomeric EMI gasket that can be
directly applied to the substrate to be shielded, thereby
eliminating the step of attaching the EMI gasket to the workpiece
at the assembly plant. One problem with the FIP process is that it
is necessary to have relatively complex and expensive dispensing
equipment at the casting or molding plant, or at the assembly
plant. As the capacity utilization of this equipment may be quite
low, due to the use on a single component this is a risky and
potentially uneconomic situation.
The second development in the area of EMI shielding is the
production of conductive coatings, especially an extensible
conductive coating, which is a coating with high conductivity that
can be applied to a film, or other flexible substrate, that is
later formed to a desired shape without substantial degradation of
conductivity. See, for example, U.S. Pat. No. 5,286,415 entitled
Water-Based Polymer Thick Film Conductive Ink and U.S. Pat. No.
5,389,403 entitled Water-based Polymer Thick Conductive Ink, the
disclosures of which are incorporated herein by reference in their
entirety. Acheson Colloids Company, located at Port Huron, Mich.,
has developed a product based on silver ink that when coated on a
thermoformable film, such as General Electric's Lexan.RTM., retains
high electrical conductivity even when drawn to relatively high
elongations. The thermoformable film may be formed to relatively
complex three dimensional shapes known as "cans." The
thermoformable film with extensible coating can replace
conventional metal cans, as well as conductive painting and plating
processes, used in mobile phones and other nonconductive small
enclosures. The thermoformable film and extensible coating can also
be part of larger electronic packages.
It has been discovered that thermoformable films, extensible
conductive coatings and FIP gaskets can be combined to produce
integral EMI shields which can be readily manufactured and shipped
from a centralized location to smaller assembly plants for
installation into electronic equipment.
The EMI shield is manufactured from a polymer thick film extensible
conductive coating, that retains high electrical conductivity at
high elongations, which is applied to a thermoformable film in
combination with a FIP gasket. The EMI shield and FIP gasket
provide EMI shielding of the entire interior of a given
structure.
For example, suitable thermoformable films include LEXAN.RTM. and
VALOX.RTM., manufactured by the General Electric Company,
Pittsfield, Mass. An example of a polymer thick film extensible
conductive coating is Electrodag.RTM. SP-405, manufactured by
Acheson Colloids Company, Port Huron, Mich.
Accordingly, in accordance with one embodiment, the invention is
drawn to a method for forming an EMI shield. The method includes
the steps of (a) providing a thermoformable film having a first
side and a second side; (b) applying an extensible conductive
coating to the thermoformable film; (c) cutting the thermoformable
film; (d) thermoforming the thermoformable film into a
three-dimensional shape; and (e) applying a compressible EMI gasket
to the thermoformable film, wherein steps (b) through (e) may be
performed in any order.
In another embodiment the invention is drawn to an EMI shield. The
EMI shield includes a thermoformable film having a first side and a
second side, wherein the thermoformable film is thermoformed into a
three-dimensional shape; an extensible conductive coating applied
to the thermoformable film; and a compressible EMI gasket attached
to the thermoformable film.
In yet another embodiment, the extensible conductive coating
includes an extensible film and conductive fibers. In another
embodiment the glass transition temperature of the extensible film
is lower than the glass transition temperature of the
thermoformable film.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of this invention may be better
understood by referring to the following description, taken in
conjunction with the accompanying drawings, in which:
FIG. 1 is a process diagram of an embodiment of the current
invention of a variety of methods for combining a formable
nonconductive substrate with a conductive coating and a FIP gap
gasket;
FIG. 2 is a schematic diagram of a conductive coating on a
thermoformable film;
FIGS. 3A-3C are schematic diagrams of embodiments of a simple and a
more complex thermoformed EMI shield;
FIGS. 4A-4C are process steps for contouring a thermoformable
film;
FIG. 5 is a table summarizing surface resistivity and shielding
effectiveness test results of conductive coatings made of various
conductive materials and thermoformable materials; and
FIG. 6 is a schematic diagram of a FIP process on a contoured
substrate.
DETAILED DESCRIPTION OF THE INVENTION
Examples of a process for manufacturing embodiments of EMI shields
are illustrated in FIG. 1.
In a first step, the EMI shield is manufactured from a
thermoformable film, such as General Electric's Lexan.RTM.. The
thermoformable film may be in small or large sheets or a long
continuous reel, depending on the scale of production required.
Generally, a formable film may be used and, in addition,
non-formable films may be used if the required shape is flat.
The thermoformable film is coated with a conductive extensible ink,
such as Acheson Colloids Company's Electrodag.RTM. SP-405 ink to
form an extensible conductive coating. The extensible ink may be
any extensible ink in the case of a 3-D shape, and any conductive
ink (or paint or plating) in the case of 2-D shapes. The extensible
ink can be applied to the film by a variety of printing or film
coating processes, such as flexographic printing, screen printing,
gravure printing, offset printing, letter press printing, pad
printing, slot coating, flood coating, spray coating, and jet
printing.
Depending on the configuration of the part used during the forming
process, there can be a considerable amount of elongation of the
EMI shield where geometric features of the shield are concentrated.
This in turn may put excessive stress on the extensible ink. If the
elongation of the extensible ink is too severe, this will result in
fracture of the conductive layer, which in turn leads to loss of
conductivity and loss of shielding. Ideally, the conductive layer
would be one that could be stretched infinitely over the entire
part. In practice, this is difficult as most highly conductive
materials will tend to fracture. Also, materials that are best for
stretching are generally not conductive enough to be used as
conductive shields.
In another embodiment the extensible conductive coating can be
formed from a combination of conductive fibers with an extensible
film. The extensible film can be selected from materials that, in
general, have a lower glass transition temperature than the
thermoformable film and, in one embodiment, can be a polymer. The
polymer selected for use with the conductive fibers can be very
thermoplastic, to the point of almost becoming a liquid, which
results in a combined polymer/conductive fiber layer that becomes
highly compliant to changes in geometry caused by thermoforming the
thermoformable film, while the conductive fibers continue to
interact with negligible loss of conductivity. FIG. 2 illustrates
an extensible conductive coating 20 on a thermoformable film
30.
In one embodiment the conductive fibers can be placed on the
thermoformable film and the extensible film can be placed on top of
the conductive fibers. The arrangement of the thermoformable film,
the conductive fibers, and the extensible film can be laminated to
allow the conductive fibers to integrate with the extensible film.
In another embodiment, the extensible film can be processed into
fibers which can be mixed with the conductive fibers. The mixture
of conductive fibers and the fibers from the extensible film can be
deposited on the thermoformable film at a temperature which at
least partially melts the extensible film fibers.
Materials for the conductive fibers include stainless steel fibers
from Baeckert, Naslon--SUS316L from Nippon Seisen Co. of
Osaka-City, Japan, Panex Chopped Fiber--PX33CF1000-01 from Zoltex
Corporation of St. Louis, Mo., and X-Static Silver Nylon Fiber from
Instrument Specialties of Scranton, Pa. Any fiber which is at least
about 3.175 mm (0.125 inches) long and less than about 0.254 mm
(0.01 inches) in diameter may be used, provided that the outer
surface of the fiber is coated with metal sufficient to produce
bulk resistivity of the material to less than about 50 milliohm-cm,
preferably less than about 25 milliohm-cm, more preferably less
than bout 10 milliohm-cm, as determined by Mil-G-83528 paragraph
4.6.11/ASTM 991. Pure component fibers can be used as well,
provided the bulk resistivity is below about these values. In
addition, some other conductive materials that can be used are
silver loaded particles, silver/copper flake, silver/nylon fiber,
silver carbon fibers, tin over copper flash, and tin.
Materials for the extensible film include polypropylene and
polyethylene fibers or films, both available from Dow Chemicals.
Other suitable polymers for the extensible film include
polystyrene, acrylonitrile-butydiene-styrene (ABS),
styrene-acrylonitrile (SAN), polycarbonate, polyester, and
polyamide, as long as the thermoplastic polymer has a lower glass
transition temperature than the supporting polymer shield, for
example at least about 20 degrees C. lower. Additionally, a
silicone material can also be used for the extensible film.
The extensible conductive coating can be made by blending
polyethylene and/or polypropylene fibers with the conductive fibers
and calendering or laminating the composite with the thermoformable
film. Other methods for applying the extensible conductive coating
to the thermoformable film include wet coating, carding, plating,
coating, flocking, dry laid screening, and vacuum metal/ion sputter
techniques.
Various combinations and permutations of the material for the
conductive fibers, the material for the extensible film, and the
method of applying the extensible conductive coating made from the
extensible film and conductive fibers to the thermoformable film
can be chosen to result in a desired surface conductivity and
shielding effectiveness of the EMI shield.
In some embodiments the conductive coating may be applied to both
sides of the thermoformable film. In other embodiments the
conductive coating may be applied to one side of the thermoformable
film. The conductive coating may be applied uniformly, or may be
applied in a pattern, such as a grid. In still other embodiments
the conductive coating may be applied in discrete areas or
zones.
In a second step, the resulting coated film is then cut to the
desired 2-D shape. Any cutting process known to those skilled in
the art can be used such as water jet cutting, laser cutting
die-cutting, hot wire cutting, etc. The film can be cut to produce
a single shape or a plurality of similar or different shapes, which
can be held together by sprues.
Next, in a third step, the cut film is thermoformed into the
desired 3-D shape. Any method of thermoforming known to those
skilled in the art may be used. The complexity of the 3-D shape can
vary significantly, from a simple box, formed by a single rectangle
draw, to a multi-chamber part with different chamber sizes and
depths. See FIGS. 3A-3C for examples.
One method of thermoforming, positive forming, is illustrated in
FIGS. 4A-4C. The thermoformable film 30 and the extensible
conductive coating 20 are heated by a heater 50 to soften the
thermoformable film 30 and the extensible conductive coating 20.
The thermoformable film 30 and extensible conductive coating 20 are
then applied to a mold 60 and a vacuum 70 drawn to conform the
thermoformable film 30 and the conductive coating 20 to the mold
60. Once cooled sufficiently, the contoured thermoformable film 30
and extensible conductive coating 20 are removed from the mold
60.
Lastly, a conductive elastomer gasket is dispensed onto the coated
thermoformed film in any desired pattern, using FIP dispensing
equipment described below and illustrated in FIG. 6. The FIP gasket
is typically applied about a perimeter, edge, lip, or other similar
structure; however, in more complex parts, the FIP gasket may be
applied to internal or external walls, dividers, or other similar
surfaces forming with adjoining structure in the final assembled
component or housing. The conductive elastomer gasket is then
cured, either at ambient temperature or via elevated temperatures,
for example, in a continuous oven.
In addition to using FIP methods for manufacturing the elastomer
gasket, other gaskets known to those skilled in the art for
shielding EMI can be used. For example, the gasket may be other
than conductive elastomers including, but not limited to,
metallized fabric wrapped foam gaskets, metal fingers, knitted
gaskets, a printable foamable ink, etc. In some cases, the finished
component may incorporate a separate environmental gasket, for
example a polyurethane gasket.
The finished shielding element is then shipped to the assembly
plant, where the entire shielding function is accomplished by
simply placing this single piece into an enclosure. Examples of
shielding composite cross-sections are shown in FIG. 1, FIG. 3C,
and FIG. 4.
Note that the four general process steps do not have to be
performed in this particular order and, in fact, may be performed
in any order. For example, the FIP gasket may be applied either
before or after coating, cutting, or forming. Similarly, the
coating may be applied either before or after cutting, forming, or
application of the FIP gasket.
FIG. 5 is a table which shows a summary of surface resistivity and
shield effectiveness test results for various conductive coatings.
The table shows the conductive materials, the base extensible
films, and the manufacturing methods for applying the conductive
coating to the thermoformable film. The table also shows the
thickness of the conductive coating and exemplary draw amounts of
the conductive coating. The test results of surface resistivity and
shielding effectiveness are provided for both an unformed
conductive layer, after application of the extensible conductive
coating to the thermoformable film and for a formed conductive
layer after three-dimensional forming of the EMI gasket. The test
results generally show the surface resistivity increases after the
conductive layer has been three-dimensionally formed. The test
results also generally show that the shielding effectiveness (SE)
remains relatively constant before and after being
three-dimensionally formed.
There are a number of ways to make a form in place gasket. An
example, as illustrated in FIG. 6, is an embodiment of a method 100
for manufacturing an EMI shield made of conductive particles and a
foamable mixture. In one embodiment, conductive particles 105, for
example, chopped metal fibers or metallized polymer fibers, are
added to the components of a foamable mixture. The components of
the foamable mixture can be a polyol component 110 and an
isocyonate component 115 of a urethane mixture. The polyol
component 110, the isocyonate component 115, and the conductive
particles 105 are mixed in one or more mixing heads 125 to produce
a urethane mixture with an integral network of conductive particles
120.
The urethane mixture with the integral network of conductive
particles 120 is then processed by available means to produce the
desired size and shape of a conductive EMI gasket. In one
embodiment, the urethane mixture with an integral network of
conductive particles 120, is dispensed through a nozzle 130
directly onto a surface 135 of an electrical enclosure 140 using an
xyz positioning system 145 to form the EMI gasket in place as the
mixture 120 foams and cures.
FIP EMI gaskets may be manufactured of conductive foams, where the
conductive elements are introduced into the foam matrix prior to
casting by adding organo-metallic compounds to the foam chemical
matrix, which are reduced to conductive elements during the foaming
process.
Additionally, various forms of carbon may be added to urethane foam
chemical precursors to produce foams with surface resistivities of
100 to 1000 ohms/square. These materials, however, have limited use
in EMI shielding applications, due to the relatively high
resistivity. A new process produces conductive foams which are less
than 10 ohms/square by introducing more highly conductive materials
into the foam chemical precursors, including silver-plated glass
spheres, sintered metal particles which have bulk resistivities
below about 10.sup.-5 ohm-cm (e.g. Cu, Al, Ni, Ag), and
silver-plated copper particles. Other conductive materials include
the class of non-metallic materials referred to as conductive
polymers. This would include such materials as poly-Analine.
Another method of producing conductive foam is to produce the
conductive elements in the foaming process by reacting
organo-metallic compounds during the foaming process. This is
accomplished by introducing reducing agents into one of the two or
more chemical precursors of the foam prior to foaming. One example
of these compounds is copper acetate, but any metal compound, which
is compatible with one of the chemical foam precursors, could be
used.
Examples of chemical foam systems which may be used include the
very broad range of urethane foams including polyester and
polyether types. Chloroprenes, more commonly known as neoprene
rubber foams, could also be used.
Variations, modifications, and other implementations of what is
described herein will occur to those of ordinary skill in the art
without departing from the spirit and the scope of the
invention.
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